Same problem here. i've tested with Mame4all and the games in vertical mode seem to have audio issues. When i switch the screen to horizontal mode, the audio on the same game is fine.

Tested it also with Advancemame and vertical mode and it sounded good.
My machine runs on the 0.7.10 version. I have tryed different memory splits, and overclock settings. No results. The problem seems to be with Mame4all...

Today i've tested some settings and the results are a bit better.
First i've changed the sample rate from 441000 to 22050 with Mame4All. Didn't notice any quality loss. Better performance? Not really.... I'ts hard to say when you hear the 80' bleeps and blops ;)

After that i put my money on the graphics side. I've changed the resolution to 640x480 60Mhz. And set the display rotating back to horizontal.
Made the change in /boot/config.txt
hdmi_group=2
hdmi_mode=4
display_rotate=0

The idea behind this is to rotate the games by Mame4all. Because we both noticed that the problem started with rotating on boot level.
So to do this, you'll have to change the setting ror=yes (mame.cfg)
I've also set the following settings to 'no'.
antialias=no
translucency=no
display_smooth_stretch=no

But there is 1 big problem. Mame4All does rotate the games, but not the menu it self.
So you'll have to use the AdvanceMenu as frontend. I'ts slower then Mame4all as Frontend... But it's worth to try.

To rotate AdvanceMenu to vertical, set display_orientation flip_xy mirror_x (advmenu.rc)

To add Mame4all to the AdvanceMenu (advmenu.rc);

emulator "PiMAME4ALL" generic "/home/pi/emulators/mame4all-pi/mame" "%s"
emulator_roms "PiMAME4ALL" "/home/pi/emulators/mame4all-pi/roms"
emulator_altss "PiMAME4ALL" "/home/pi/.advance/snap" (make use of the snaps provided with AdvanceMenu)
emulator_include "PiMAME4ALL"
emulator_attrib "PiMAME4ALL" missing exclude

I also made sure that my Pi runs at the preferred overclock;
arm_freq=900
core_freq=300
sdram_freq=500

At the end, some roms have better sound. But it's not perfect. I also think that to problem lies within the roms it self. I've noticed that Ms.Pacman with rom version 1.49 on my PC has a far better sound... But running more updated roms with Mame4all is a no-go...

Let me know what the outcome is with the USB sound card

I have a spare SDcard with RetroPie on it. Never tested it with my speakers hooked up, so i just did :)

Used the same roms as i used on Pimame 0.7.10 version and left everything, except for display rotating from /boot/config.txt to vertical, at default. And the issues with the sound are the same as with the default settings from my Pimame image.
The settings as described above seems to have better results.

if you want, i can spit out the complete settings for the 3 files. If it's possible i could also share the image from my current SDcard so you can try it for yourself. Only problem is how to share a 4gig file :)

I'm still searching for a solution. But i think that the problem lies within the roms or Mame4all/AdvanceMame. Today i'm trying some settings with the alsa-driver. The weather forecast for today, in the Netherlands, isn't that great. So i stay mostly inside and try to find a solution...

I will give it a try today on a spare SDcard.
For now it seems to be the problem with Mame4all. I have tested my short list of games for my Bartop with AdvanceMame and the games that did work had better audio. The games that didn't work with rom version 0.37b5, i replaced with rom version 1.49. I also tryed some bootlegs and other clone versions. Now i have a mix of mostly games running with AdvanceMame, except for 5 roms that didn't work, but did work with Mame4all.

For now, my machine runs with AdvanceMenu and a mix of AdvanceMame and Mame4all. There is still a problem with audio from AdvanceMenu. So for now i've turned audio for the menu off...